Small size ROM

ABSTRACT

The invention concerns a ROM circuit ( 40 ) comprising columns of storage cells, each column being connected to a bit site (BLi, BLi+1), wherein the columns are arranged in groups of two adjacent columns, each column of a group capable of being selectively activated relative to the other column of the group, thereby enabling the elimination of a connection to the ground of columns and the design of efficient reading amplifiers.

[0001] The present invention relates to non-volatile memories, also called ROMs (read-only memories).

[0002] In a ROM, the information is stored once and for all upon manufacturing of the memory. Such a memory includes several blocks of memory cells organized in columns and rows, and can only be used for reading.

[0003] In FIG. 1, a block of a ROM 10 includes memory cells 1 forming two columns A_(i) and A_(i+1), respectively connected to bit lines BL_(i) and BL_(i+1). Memory cells 1 of the shown block are also arranged according to sixteen rows, a memory cell being present at each intersection of a column and of a row. Each memory cell 1 includes or not a transistor 2. The three gate, drain, and source electrodes of each transistor 2, are respectively connected to a word line WL₀ to WL₁₅, to a bit line BL_(i), BL_(i+1), and to ground (GND). When cell 1 includes no transistor 2, the corresponding word line simply crosses the column with no action thereupon.

[0004] Each column also includes a selection transistor 5 controlled by an activation line BS (“Block Select”) common to all the columns in the block. When the activation line receives a high voltage, generally supply voltage VDD, also corresponding to a logic “1”, all transistors 5 of the block columns are on, which puts in communication each column of the block with its respective bit line. An amplifier 6 connected to each bit line BL_(i), enables reading the stored information.

[0005] It should be noted that “column” here designates the assembly formed by a selection transistor 5 and all the memory cells coupled to this selection transistor. Each column has an end connected to a bit line. The other end of each column is grounded, either via the source of the transistor of the last memory cell, if said cell includes one (case of column A_(i)), or directly if the last cell includes no transistor (case of column A_(i+1)). It should also be noted that bit lines BL_(i), BL_(i+1) continue beyond the shown block for connection to other columns of other blocks of memory 10.

[0006] For reading from a row of a block of memory 10, the bit lines are first precharged to a high voltage, generally supply voltage VDD, and activation line BS of the block is brought to “1” (VDD). Then, the word line of the selected row is set to “1”, the other word lines being at “0” (GND). All the transistors of the unselected rows, receiving a null voltage on their gates, will be off and will have no influence upon the read information. However, all the transistors in the selected row will be on, which results in discharging the bit line to which they are connected. Thus, if the selected cell 1 includes a transistor 2, the bit line is discharged and a “0” is read by amplifier 6. If the cell includes no transistor 2, the bit line will not be discharged and a “1” will be read at the output of amplifier 6. For example, if line WL₁₅ is selected, amplifier 6 of bit line BL_(i) will provide a “0” and the amplifier of bit line BL_(i+1) will provide a “1”.

[0007]FIG. 2A illustrates another type of ROM 20. In memory 20, the memory cell columns will be formed of an assembly of transistors in series. Each column A_(i), A_(i+1) includes a selection transistor 5 enabling activation of the column to which it belongs and transistors 10 corresponding to the memory cells. One end of each column is connected to a bit line BL_(i), BL_(i+1), and the other end of the column is connected to ground (GND). To represent a “0”, transistor 10 is short-circuited by a conductive link 11 between its source and its drain. To represent a “1”, transistor 10 is not short-circuited. Each selection transistor 5 is controlled by an activation line BS common to all columns in the block, and each bit line is connected to a read amplifier 6. The bit lines continue, beyond the shown block, towards other memory blocks.

[0008] To read from a row of a block of memory 20, the bit lines are first precharged to a high voltage, generally VDD, and activation line BS of the block is brought to “1” (VDD). All transistors 5 of the considered block are then on. Then, a row is selected by bringing the corresponding word line to a low potential, generally the ground potential, corresponding to logic “0”. All the other word lines are brought to a high potential. All the transistors in the unselected rows are then on and present no obstacle to the passing of a current through the column. The transistors of the selected row, having their gate to potential 0, are all off. If the transistor is not short-circuited, as is the case for transistor 10 located at the intersection of bit line BL_(i) and of word line {overscore (WL)}₁₅, the transistor is off and it will prevent the bit line discharge. The corresponding bit line will thus remain at the high potential and the corresponding read amplifier 6 will provide a “1”. However, if the transistor is short-circuited, as is the case for the transistor 10 located at the intersection of bit line BL_(i+1) and of word line {overscore (WL)}₁₅, the fact that this transistor is off does not disturb the current conduction in the column connected to bit line BL_(i+1) which will discharge to ground via all the other on transistors in the column. The corresponding amplifier 6 will thus read a “0”.

[0009] The circuit of FIG. 2A is often preferred since it is easy to implement in integrated technology.

[0010]FIG. 2B shows a simplified top view of memory 20 of FIG. 2A, made in the form of an integrated circuit. Memory 20 is made on a substrate 21 including two elongated active areas 22 and 22′. Areas 22, 22′, for example N doped, are insulated from each other and respectively correspond to the drains and to the sources of series transistors 10 of columns A_(i), A_(i+1) of FIG. 2A. Polysilicon strips 24 pass on active areas 22 and 22′ while being insulated therefrom. Strips 24 form word lines {overscore (WL)}₀ to {overscore (WL)}₁₅. They form, at each of their intersections with areas 22, 22′, the gate of a transistor 10. This results in the forming of a chain of transistors 10 in series, along areas 22, 22′. For each block, a specific strip 24′ forms activation line BS and forms selection transistor 5. Each column includes, at one end, a contacting area 25 _(i), 25 _(i+1) for the corresponding bit line and, at the other end, a contacting area M for connection to the ground. When a conductive link must be formed to short-circuit a transistor, as is the case for the transistor 10 located at the intersection of word lines {overscore (WL)}₁₅ and of active area 22′, contacting areas 26 and 26′ are provided on either side of the gate of the considered transistor for connection to an upper metallization level.

[0011] ROMs such as illustrated in FIGS. 1, 2A, and 2B have disadvantages.

[0012] For example, the ground contact, which is at one end of the active area generally requires an additional metallization level, which increases the size, the manufacturing times and the costs.

[0013] Also, the quality of the read amplifiers is mediocre. As they are located at the end of the bit lines, the read amplifiers can indeed only occupy the width thereof. They are accordingly small and cannot be very elaborately designed. For example, their response time is low, which limits the maximum frequency of use of the memory, and they consume significant power.

[0014] An object of the present invention is to provide a ROM-type memory of reduced size.

[0015] Another object of the present invention is to provide a ROM including sophisticated read amplifiers having reduced power consumption.

[0016] To achieve these objects as well as others, the present invention provides a ROM circuit including memory cell columns, each column being connected to a bit line, in which the columns are arranged in groups of two adjacent columns, each column in a group being selectively activable or inactivable with respect to the other column in the group by means of an activation line, characterized in that each column in a group is connected by one end to the activation line of the other column in the group.

[0017] According to an embodiment of the present invention, the activation line of a column is brought to the ground potential to deactivate said column.

[0018] According to an embodiment of the present invention, a column comprises a plurality of memory cells in series, each memory cell comprising a MOS transistor, the drain, respectively the source, of which is coupled either to the source, respectively the drain, of an adjacent memory cell, or to an end of the column.

[0019] According to an embodiment of the present invention, each column of a group comprises a selection means capable of selectively activating/deactivating said column, controlled by the activation line of the column.

[0020] According to an embodiment of the present invention, the selection means of a column comprises a MOS transistor in series with the memory cells of the column and arranged at the end of the column not connected to the activation line of the other column of the group.

[0021] According to an embodiment of the present invention, the circuit includes an amplifier connected to the bit lines connected to the two columns of a same group.

[0022] According to an embodiment of the present invention, the amplifier includes a means for invalidating the information present on the bit line connected to the deactivated column in the group.

[0023] According to an embodiment of the present invention, the amplifier includes a means for lowering the voltage present on the bit line connected to the deactivated column in the group.

[0024] The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which:

[0025]FIG. 1, previously described, schematically shows a first example of a conventional ROM;

[0026]FIGS. 2A and 2B, previously described, schematically show, respectively, the electric diagram and a top view in integrated form of a second example of a conventional ROM;

[0027]FIG. 3 schematically shows a first embodiment of the present invention; and

[0028]FIG. 4 schematically shows a second embodiment of the present invention.

[0029] The same elements have been referred to with the same references in the different drawings.

[0030] According to the present invention, the columns of a ROM circuit are arranged in groups of two adjacent columns. Each of the columns of a group includes a selection means, controllable by a specific activation line. The activation line enables selection either of all the columns of even rank, or of all the columns of odd rank of a memory block. According to the rank of the selected columns, data coming either from all the bit lines of even rank, or from all the bit lines of odd rank, will be obtained in read mode.

[0031] In FIG. 3, a ROM 30 is shown with two adjacent columns A_(i), A_(i+1), respectively connected to bit lines BL_(i) and BL_(i+1). Each column includes transistors 10, short-circuited or not, corresponding to the memory cells. Column A_(i) further includes, at one end, a selection transistor 35, controlled by an activation line BS_(i). Column A_(i) is connected to bit line BL_(i) by the end including transistor 35. Similarly, column A_(i+1) includes, at one end, a selection transistor 36 controlled by an activation line BS_(i+1). Column A_(i+1) is connected to bit line BL_(i+1) by the end including transistor 36. Further, the end of column A_(i) that does not include transistor 35 is connected to activation line BS_(i+1) and the end of column A_(i+1) that does not include transistor 36 is connected to activation line BS_(i).

[0032] According to the present invention, the two columns A_(i), A_(i+1) form a group of two selectively-activated adjacent columns, that is, when a column in the group is activated, the other one is deactivated, and vice versa. For this purpose, transistors 35 and 36 are selectively controlled, that is, activation lines BS_(i) and BS_(i+1) receive complementary signals. Thus, when activation line BS_(i) is at “1”, activation line BS_(i+1) is at “0” and transistors 35 and 36 are respectively on and off. Only column A_(i) is then activated. Indeed, the on transistor 35 puts in communication the memory cells of columns A_(i) with bit line BL_(i) and the end of column A_(i) opposite to transistor 35 is at the ground potential, since it is connected to BS_(i+1), then at “0”. Conversely, when activation line BS_(i+1) is at “1”, line BS_(i) is at “0” and only column A_(i+1) is activated.

[0033] Thus, in the memory of FIG. 3, the setting to a null potential of the end of the columns is performed by connecting the end of a column of a group to the activation line of the other column in the group. This is possible according to the present invention since, when a column of a group is activated, the voltage on the corresponding activation line is at VDD and the voltage on the activation line of the other column in the group is equal to zero. Accordingly, the connection of the source of the last transistor of a column in a group to the activation line of the selection means of the other column in the group does not modify the memory operation. The ground connection of the columns has become useless. This can avoid an additional metallization layer and make the memory more compact while decreasing its manufacturing costs.

[0034] To easily make the connection of the last transistor in a column to the activation line of the other column in the group, the selection means of the second column in a group will preferably be located on the side opposite to the selection means of the first column, as illustrated in FIG. 3. However, as an alternative, it is possible to eliminate a ground connection by arranging the two activation lines BS_(i) and BS_(i+1) side by side, those skilled in the art defining the modifications to be made to the diagram of FIG. 3.

[0035] The fact of providing the grouping by two of the adjacent columns and of selectively activating them also enables having read amplifiers with higher performance.

[0036]FIG. 4 illustrates a second embodiment of the present invention. In FIG. 4, a ROM 40 includes the main elements of memory 30. Further, an amplifier 41 replaces the two read amplifiers 6 of FIG. 3. Amplifier 41 has two inputs, respectively connected to bit lines BL_(i) and BL_(i+1).

[0037] The replacing by amplifier 41 is made possible by the grouping by two of the adjacent columns. Indeed, in read mode, a single one of the two columns in a group is activated and, accordingly, a single read amplifier is necessary per group.

[0038] Read amplifier 41 can thus occupy up to two times as much room as each of prior art amplifiers 6. In practice, it will generally be a little smaller than two amplifiers 6, which saves space. Its design can be neater and, in particular, the frequency response of the amplifier can be faster, which, as it decreases its response time, enables increasing the maximum frequency of use of memory 40. Since it is faster, amplifier 41 can further be activated for a shorter duration in read mode and it will consume less power. Since there are many read amplifiers in a memory, for example 512 or 1024 per block, the present invention also enables significantly decreasing the power consumption of the memory.

[0039] Amplifier 41 can operate in various ways. For example, the input corresponding to the deactivated column can be invalidated, the amplifier only effectively receiving the information to be read.

[0040] Thus, amplifier 41 can include differential stages coupled to the two bit lines BL_(i) and BL_(i+1), which then perform a complementary function, somewhat like in the case of a DRAM. In a read operation, the bit lines are first precharged to any voltage, for example VDD. Then, the selected column in the group provides a “1” or a “0” and, according to the case, the corresponding bit line remains precharged at VDD or discharges to 0. For the differential stages of amplifier 41 to properly discriminate a 1, the voltage will have to be lowered on the bit line of the deactivated column, for example by being brought down to VDD/2. Such a modification is within the abilities of those skilled in the art and will not be detailed any further.

[0041] Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art.

[0042] Thus, the high voltage has been described as being a supply voltage VDD, but another value may be used. Similarly, the low potential has been described as being the ground potential, but it can be a virtual ground associated with the memory, distinct from the ground of other circuits linked to the memory.

[0043] Also, the functions of the high and low voltages and of the logic “1s” and “0s” may be inverted without departing from the field of the present invention.

[0044] Also, even though the described embodiments provide arranging the columns of the blocks of a ROM in groups of two adjacent columns, it is within the abilities of those skilled in the art to arrange the columns in groups of any number n of adjacent columns, n selection means then enabling activation of a single column at a time. 

1. A ROM circuit including memory cell columns, each column being connected to a bit line, wherein the columns are arranged in groups of two adjacent columns, each column in a group being selectively activable or inactivable with respect to the other column in the group of an activation line, wherein each column in a group is connected by one end to the activation line of the other column in the group.
 2. The memory circuit of claim 1, wherein the activation line of a column is brought to a ground potential to deactivate said column.
 3. The memory circuit of claim 1, wherein a column comprises a plurality of memory cells in series, each memory cell comprising a MOS transistor, a drain, respectively a source, of which is coupled either to a source, respectively a drain, of an adjacent memory cell, or to an end of the column.
 4. The memory circuit of claim 3, wherein each column of a group comprises a selection means capable of selectively activating/deactivating said column, controlled by the activation line of the column.
 5. The memory circuit of claim 4, wherein the selection means of a column comprises a MOS transistor in series with the memory cells of the column and arranged at the end of the column not connected to the activation line of the other column of the group.
 6. The memory circuit of claim 1, including an amplifier connected to the bit lines connected to the two columns of a same group.
 7. The memory circuit of claim 6, wherein the amplifier includes a means for invalidating the information present on the bit line connected to the deactivated column in the group.
 8. The memory circuit of claim 6, wherein the amplifier includes a means for lowering the voltage present on the bit line connected to the deactivated column in the group. 